As a former sailor, I will offer that military personnel, ideally, know which one of a set of roles they are filling at the moment. It's a system of abstraction designed to simplify decision making when complex, fast moving situations arise. Each person knows several roles, and each role has a scope that it concerns itself with.
For instance - sniper teams consist of a spotter and a shooter. Between them they have a litany - specific things they say and do. Usually these roles are reversible, even though one of them is probably a better shot.
Even in simple guard duty, there will be a structure with defined roles. Not only will you have the guys at the gate and the patrols, but you'll have dispatch radio all points in a specific known order, with team names you won't find laying around if you just wiped out a patrol. On high alert, this round robin might never stop, or they'll have full duplex gear (everyone can hear everyone else all the time). Besides those guys and the watch commander, you'll have a ready reserve team with a specific response plan and timing requirements that they train on frequently. There will also be a duress code - something they can say that sounds totally normal but tells everyone that can hear it that there is a security breach in progress.
I will also tell you that your sense of tactics can be greatly informed by simply running little scenarios in your head. On the submarine we were very fond of discussing first how to most effectively steal or destroy our own ship, followed by how to effectively detect and prevent those things supposing we had no prior knowledge. We decided that under no circumstances should we ever allow someone on board carrying anything in a bag unless their orders were known in advance, and "administrative errors" would be searched before making calls to the squadron building - this turned into 100 percent bag checks.
When you, as the GM, run these NPCs, ask yourself - what role is this person filling? Why is he standing there, with a weapon, and what is he worried about? What tools are available to him, and how will he respond when someone tries to hurt him?